bus_if: provide a default null rescan method

[mhorne]

Summary

There is an existing helper method in subr_bus.c, but almost no drivers
know to use it. It also returns the same error as an empty method,
making it not very useful. Move this to bus_if.m and return a more
sensible error code.

This gives a slightly more meaningful error message when attempting
'devctl rescan' on buses and devices alike:

"Device not configured" --> "Operation not supported by device"

[imp]

Not sure about the ofw_pcibus.c changes, but the overall change is good, and the cardbus bits are also: There's no need to ever rescan cardbus since we always know (we have a polling backup in case the admin interrupts on that interface are wonky). Since we look so often, rescan will always be a nop.

[mhorne]

In D35501#805061, @imp wrote:

Not sure about the ofw_pcibus.c changes, but the overall change is good, and the cardbus bits are also: There's no need to ever rescan cardbus since we always know (we have a polling backup in case the admin interrupts on that interface are wonky). Since we look so often, rescan will always be a nop.

Thanks. I did at least look at the callers of BUS_RESCAN() and I did not see anything that depended on a particular error value being returned.